1. Field of the Invention
The present invention relates to rain sensors, and in particular, to an optical rain sensor that is mounted on the inner surface of a vehicle windshield to detect raindrops adhered to the outer surface of the vehicle windshield.
2. Description of the Related Art
Generally, in optical rain sensors, a light beam emitted from a light source such as a light emitting diode (LED) is incident on the inner surface of the windshield glass via a prism. The light beam reflected from the outer surface of the windshield is collected by a prism and then is received by a photoelectric transducer such as a photodiode. Since the detection signal from the photoelectric transducer is very weak, the detected voltage is usually amplified in an amplifier circuit, and then the amplified output voltage is input to a calculation circuit. After the output voltage from the amplifier circuit has been raised to a predetermined voltage level, for example, 3.0V for analog/digital (A/D) conversion, the sensor detects raindrop precipitation based on the magnitude of decrease (or a decrease ratio) in output voltage from this predetermined voltage.
The sensor sensitivity to raindrops is significantly dependent on the area of the outer windshield surface (detection area) onto which the sensor light beam is irradiated. The larger the detection area, the easier the sensor detects raindrops. This is because the larger the detection area, the more likely raindrops will hit this detection area.
If the detection area is enlarged, however, the sensor body becomes large and may obstruct the driver""s view because the sensor is installed near the rearview mirror installation location. At the same time, the sensor cost will increase and become high.
Conventionally, the threshold value for detecting precipitation has been raised with a software program, so that rainfall can be detected even by a weak sensor signal. However, such a software-based adjustment for higher sensitivity is subject to constraints of the resolution of the employed A/D converter. As a result of the limited sensitivity, it has been difficult to detect rainfall with a high level of sensitivity.
It is therefore an object of the present invention to solve the above-mentioned problems by providing a rain sensor that has improved sensitivity while being simple in structure, compact in size, and having a low manufacturing cost.
A rain sensor has an amplifier circuit that amplifies an output voltage from a photoelectric transducer and provides this output voltage signal to a calculation circuit. The rain sensor raises the output voltage from the amplifier circuit up to a predetermined level and detects rainfall based on the drop in the output voltage from the predetermined level. The present invention provides an improvement that the amplifier circuit conducts an offset amplification.
According to the rain sensor of this invention, the offset amplification augments the output voltage drop from the predetermined value, even when the intensity of rainfall is the same. As a result, it becomes possible to raise sensor sensitivity while maintaining sensor compactness and a low manufacturing cost.
If the offset level in such offset amplification is variable, it becomes possible to raise sensor sensitivity by increasing the offset level, for example, during periods when the driver runs the car at night or at high speeds. In turn, when the car is at a standstill, the sensor sensitivity may be lowered by decreasing the offset level. In this manner, the sensor sensitivity becomes controllable in the present invention.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.